As is known in the art, Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays. UV light includes wavelengths in the range of 100 to 400 nanometers (nm) and energies from 3 to 124 electron-volts. UV light is emitted by the Sun, electric arcs, and artificial light sources such as black lights. As an ionizing radiation, UV sources can cause chemical reactions that cause many substances to glow or fluoresce. Although UV light is beyond visible light, most people are aware of the effects of UV radiation through sunburn. However, the UV spectrum has many other effects, both beneficial and damaging, on human health.
UV radiation can be classified in a number of ways, including as bands of radiation separated into the following:
UV A-band: 400-320 nm;
UV B-band: 320-280 nm; and
UV C-band: 280-100 nm.
The Sun emits UV A, UV B, and UV C-band radiation; however, the Earth atmosphere's ozone layer absorbs about 98.7% of this radiation, primarily in the UV B and UV C-bands. Other natural sources of UV radiation include lightning and fires.
As is also known in the art, ordinary glass is partially transparent to UV A-band radiation, but is opaque to shorter UV wavelengths in the UV B and UV C-bands. Silica or Quartz glass, depending on quality of materials (lack of impurities), can be transparent to UV C-band radiation, while window glass passes about 90% of UV light above 350 nm, but blocks over 90% of UV light below 300 nm.
As is also known in the art, UV C-band radiation is invisible to humans and to ordinary cameras, video gear, and night vision systems. Existing UV detectors generally include either solid-state devices, such as a silicon carbide or aluminum nitride device, or gas-filled tubes as sensing elements. UV detectors are primary used for detecting artificial light. For example, the petrochemical industry uses UV detectors to monitor burning hydrogen flames which radiate strongly in the 185-260 nm range. UV detectors are sensitive to burning compounds, such as hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Industrial safety applications employ UV detectors to effectively detect arc-welding, electrical arcs, lighting, and X-rays.
UV C-band cameras have been used to detect invisible coronas emanating from hot spots in high voltage power lines to identify potentially destructive electrical arcs (so called “flash-overs”). These cameras use special quartz or Calcium Fluoride lenses and have split optical paths to process and eventually combine visible and UV light into a visible image. The visible light follows a conventional video-camera path, while the UV light passes through a UV C-band transmissive optical path. The UV light then passes through a solar blind filter into a UV Intensified Charged Coupled Device (ICCD). Image recombination results in invisible UV radiation being rendered as visible light as an overlay on a standard video image. Typically, camera operators can see a bright cloud of UV C-band photons when viewing a UV C-band emissions source through a live UV C-band camera.